This invention relates generally to xerographic printing, and more specifically to a xerographic machine having an impulse air ejector cleaning system for effectively removing airborne particles that may otherwise adhere to critical xerographic imaging elements of the machine, thus causing copy quality defects.
In the well-known process of xerographic or electrostatographic printing, a charge retentive surface, typically known as a photoreceptor, is electrostatically charged, and then exposed to a light pattern of an original image to selectively discharge the surface in accordance therewith. The resulting pattern of charged and discharged areas on the photoreceptor form an electrostatic charge pattern known as a latent image. The latent image is developed by contacting it with a developer material consisting for example of carrier particles and powder-like toner.
The toner is attracted and held onto the image areas by electrostatic charge on the photoreceptor surface. Thus, a toner image is produced in conformity with a light image of the original being reproduced. The toner image is then transferred to a copy sheet, and affixed thereto to form a permanent record of the image to be reproduced. Following such development and transfer, excess toner left on the photoreceptor is cleaned from its surface. The process is useful for light lens copying from an original document, for scanned copying, or for printing electronically generated or stored originals such as with a raster output scanner (ROS), where a charged surface may be imagewise discharged in a variety of ways.
The foregoing discussion generally, describes a typical black and white or single color electrostatographic printing process. The approach utilized for multicolor electrostatographic printing is substantially identical. However, instead of forming a single latent image on the photoreceptor, multiple latent images corresponding to different color separations are sequentially recorded on the photoreceptor. Each single color latent image is developed with toner having a complimentary color. This process is repeated for each of the differently colored images with a respective toner of a complimentary color. Thereafter, each single color separation toner image is transferred to the copy sheet in superimposed registration with the prior color separation toner image, thus resulting in a multilayered, multicolor toner image. This multi-layered, multicolor toner image is then transferred and permanently affixed to a copy sheet in a conventional manner, in order to form a finished color copy.
A constant problem in a xerographic or electrostatographic printing machine as described above which uses dry powder toner, is the detrimental effects of airborne toner and dust particles (created during operation), on critical xerographic elements of the machine. These airborne particles also include loose particles from a document handler, degraded portions of a doctor blade or transfer roll, and the normal dust and dirt from the surrounding environment. Image quality degradation typically results when airborne particles adhere for example to the surfaces of critical components such as the optical components used to discharge areas on the photoreceptor, and on sensors.
Optical components, such as a ROS (raster output scanner) are arranged along an optical path and include mirrors and lenses. Over time, they may acquire a sufficient layer of particles so as to reduce exposure, at the photoreceptor, by partially blocking light reflected from or transmitted through them. The particles can also reduce contrast, in an image exposure profile at the photoreceptor, by scattering light reflected from a mirror component. This may produce dark lines in light areas in conventional charged area development systems, or it may produce light streaks in imaged areas in systems employing discharge area development, the lines in both cases being aligned in the direction of the photoreceptor motion.
Various methods of reducing airborne particle contamination in xerographic printing machines are known in the art. For example, it is well known to provide a continuous, low pressure positive air flow across critical xerographic elements or components so as to prevent particle adhesion at their surfaces. Another technique involves isolating the component for example by placing it inside a housing. However, airborne particles may still affect the cleanliness and effectiveness of such components because turbulent air flow within the machine tends even to allow airborne particles to enter into such housings.
The following briefly summarized additional disclosures may be relevant to various aspects of the present invention. U.S. Pat. No. 5,570,161 for example describes a method for reducing the rate by which airborne particles are deposited on the surface of optical components contained within the housing of a ROS. The lenses, mirrors, and transparent exit window are coated with a low energy material to minimize the Van der Waal and capillary forces that cause small particle adhesion. Electrostatic charge build-up, which attracts larger particles, is reduced by modifying the lateral conductivity of the coating. A fluorinated carbon film is applied to the coating to dissipate the surface charge. Overall efficiency in removing a particulate layer is increased by the addition of air assisted cleaning.
U.S. Pat. No. 5,613,174 describes an air moving device that is coupled to the housing of a ROS for maintaining an outwardly directed flow of air from an open end of the housing so as to move airborne particles away from the open end of the housing. An electrically biased member is located between the ROS and a toner particle carrying surface for attracting toner particles, thus preventing such particles from contaminating the ROS.
Generally thus, it can be seen that xerographic components conventionally, are cleaned or protected against airborne toner particles and other airborne contaminants by using a system of continuous low pressure air that is set to gently blow on critical xerographic components without disturbing their operation. This continuous low pressure air system usually blows approximately one liter of air per minute (or less than one cfm of air) continuously into a component such as an electrostatic voltmeter (ESV) or a patch generator sensor. At best, such conventional continuous low pressure air systems work only marginally. This is because any disturbance of a sensitive component due, for example, to a sudden increase or change in the level of continuous air pressure would likely result in erratic measurements by such component.
For example, such a disturbance likely will result erratic readings of the voltage level on photoreceptor surface. In addition, the continuous blowing of air has to kept at such a low level that is unlikely to disturb toner images on the photoreceptor surface. Unfortunately, however, such a low level of continuous air pressure is also unlikely to blow off some particles on surfaces of critical components that are adjacent to the photoreceptor surface. A good amount of such particles actually settle on these is surfaces during idle periods of the machine, and a gentle continuous air blowing on them after the machine starts up would likely not be enough to purge or remove the particles from such surfaces.
When such components become significantly dirty thus, they invariably start to produce signals that are inaccurate, in turn, affecting other subsystems and controls within the machine. Images produced are detrimentally affected, becoming either too light or too dark, and often having poor quality backgrounds, thus requiring an expensive service call.